The prior art has already dealt in an extensive way with the identification of security documents.
Some prior art solutions go in the direction of recognition of possible characteristic patterns printed at the surface of some security documents.
In order to prevent genuine security documents from being falsely copied by means of high-resolution colour photocopying apparatus, the prior art has also proposed to add to the fibrous structure of the base material of the substrate or to the surface of the document one or more security elements allowing the identification and/or making difficult the manufacturing of the document.
U.S. Pat. No. 4,114,032 (priority date 1973) and U.S. Pat. No. 4,218,674 (filing date 1975) disclose a similar system where the security documents have fibres which are coated with a magnetic or magnetizable material embedded therein. The mere presence of the magnetic fibres inside the security documents is tested or, as an improved feature, the distribution of the magnetic fibres in the security document is measured so that every single security document can be given a unique mark.
Up to 500 million different possible combinations may be obtained. EP-A-0 625 766, EP-A-0 632 398 and EP-A-0 656 607 (all filing date in 1993) disclose a system where the fibres consist of magnetic powder as core of a polymer sheath. Magnetic detection is done by DC current used to excite a coil. Because of magnetic prehistory or disturbing of magnetic fields or deformations of the security documents, however, the repetitivity of such a magnetic scanning system is not ensured and accurate discrimination between genuine security documents and counterfeit documents is not always guaranteed. So detection is not always distinctive.
Moreover, if characters on the security document have been printed by means of a magnetic ink which is detectable by means of a sorting apparatus, there may be interference between the magnetic fibres and the magnetic ink of the characters.
Other embodiments disclosed in the prior art are based on the detection of particular electromagnetic properties of the security elements. FR 2 425 937 discloses a method of dispersing metallic fibres, more particularly stainless steel fibres, inside the fibrous structure of paper in order to allow the identification by means of microwaves.
U.S. Pat. No. 4,820,912 (priority date 1985) discloses an alternative system where the security documents comprise randomly distributed electrically conductive fibres. By scanning the documents by means of microwaves the unique distribution of the fibres inside the security document can be obtained. Up to 64.sup.320 different possible combinations of the mark characterising this distribution can be obtained. Application of this microwave technique to reproduction apparatus such as photocopying apparatus in order to prevent security documents from being copied, such as disclosed in WO-A- 95/24000 (priority date 1994) fails to distinguish security documents from printed circuit boards (PCB's) or from greeting cards having decorative metal foils on its surface. On the other hand, the system does not discover the presence of the fibres if a metal plate is put above a genuine security document. Particular cover lids of photocopying apparatus or metallic parts in the neighbourhood of the photocopying apparatus, may disturb the system. As a consequence, these systems are not completely reliable.
The prior art has also provided a number of optical authentication systems. Some of them have been disclosed already in U.S. Pat. No. 3,313,941 (filing date 1963) and in U.S. Pat. No. 3,449,585 (filing date 1966). All optical systems, however, suffer from the major drawback that wear or damage or dirt on the surface of genuine security documents can cause the security documents as being no longer recognised as authentic.